Widely tunable infrared source system and method

ABSTRACT

A system and method for tuning and infrared source laser in the Mid-IR wavelength range. The system and method comprising, at least, a plurality of individually tunable emitters, each emitter emitting a beam having a unique wavelength, a grating, a mirror positioned after the grating to receive at least one refracted order of light of at least one beam and to redirect the beam back towards the grating, and a micro-electro-mechanical systems device containing a plurality of adjustable micro-mirrors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/923,344, filed Jun. 20, 2013, which claims the benefit of andpriority to U.S. Provisional Patent Application No. 61/661,836, filedJun. 20, 2012, the entire disclosure of each of which is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present embodiments relate generally to laser systems and moreparticularly to widely tunable infrared source (WTIRS) laser systems andmethods.

2. Description of the Prior Art

Widely tunable infrared source lasers are a special class of Wavelengthbeam combining (WBC) lasers. WBC methods have been developed to combinebeams along a combining dimension and produce a high powermulti-wavelength output.

There are various known methods for making tunable diode orsemiconductor lasers. These methods are: Littrow, Littman-Metcalf, andsampled grating. In both Littrow and Littman-Metcalf configurationswavelength tuning is accomplished by mechanically rotating thediffraction grating or mirror. However, there are disadvantages withthese methods. For example, the tuning range for such a tunable laser islimited to the gain bandwidth of each diode emitter. Additionally, thewavelength tuning speed is very slow and is limited by the mechanicalnature of the tuning mechanism. In a sampled grating approach, thetuning speed can be very fast. However, the tuning range is limited tothe gain of the diode element.

The following application seeks to solve the problems stated.

SUMMARY OF THE INVENTION

Disclosed herein is a system and method for tuning an infrared sourcelaser in the Mid-IR wavelength range. The system and method comprising,at least, a plurality of individually tunable emitters, each emitteremitting a beam having a unique wavelength, a grating, a mirrorpositioned after the grating to receive at least one refracted order oflight of at least one beam and to redirect the beam back towards thegrating, and a micro-electro-mechanical system (MEMS) device containinga plurality of adjustable micro-mirrors.

In at least one embodiment, a cavity consists of an individuallyaddressable diode or QCL laser array, a transform lens, a diffractiongrating, a second transform lens, and a digital micromirror device(DMD). In some embodiments, the cavity may be a conventional WBC.

In one exemplary embodiment, the QCL or diode array may consist of 20emitters, each emitter having a previously specified gain peak. In suchan embodiment the first emitter may have a gain peak at 6 μm and theadjacent emitter has a gain peak at 6.2 μm, wherein the gain peak ofeach element thereafter increments by 0.2 μm. Thus, enabling discretewavelength tuning by switching on/off specified DMD mirrors. A DMD chipmay have on its surface several hundred thousand microscopic mirrorsarranged in an array which correspond to the pixels in an image to bedisplayed. The mirrors may be individually rotated ±10-12°, to an on oroff state with the light being reflected to a beam dump. In the on statethe light is stabilized and exits the system as a stabilized wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a combining schematic of previous a WBC system.

FIG. 2 illustrates a widely tunable infrared source laser system.

FIG. 3 illustrates the efficiency of at least one embodiment of grating,150 l/mm, used on a widely tunable infrared source laser system.

FIG. 4 illustrates the tuning range of a 5 micron wavelength quantumcascade laser.

FIG. 5A illustrates the L-I-V characteristics of a mid-wavelengthinfrared (MWIR) quantum cascade laser (QCL) at 4.9 center wavelengthunder continuous wave (CW) and pulsed operation of a widely tunableinfrared source laser system.

FIG. 5B illustrates the wall-plug efficiency of an MWIR QCL at 4.9center wavelength under CW and pulsed operation of a widely tunableinfrared source laser system.

FIGS. 6A and 6B illustrate the L-I-V characteristics of along-wavelength infrared (LWIR) quantum cascade laser (QCL) atapproximately 10.2 microns center wavelength under pulsed operation(FIG. 6A) and continuous wave operation (FIG. 6B) of a widely tunableinfrared source laser system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For purposes of this application optical elements may refer to any oflenses, mirrors, prisms and the like which redirect, reflect, bend, orin any other manner optically manipulate electromagnetic radiation.Additionally, the term beam includes visible light, infrared radiation,ultra-violet radiation, and electromagnetic radiation. Emitters includeany beam-generating device such as semiconductor elements, whichgenerate a beam, but may or may not be self-resonating. These alsoinclude fiber lasers, disk lasers, non-solid state lasers and so forth.Generally each emitter is comprised of at least one gain element. Forexample, a diode element is configured to produce a beam and has a gainelement, which may be incorporated into a resonating system.

It should also be understand that certain emitters mentioned inembodiments below, such as a diode element, may be interchanged withother types of beam emitters.

DLP in industry is sometimes used to mean an array of individuallycontrollable micromirrors, because these chips are sometimes called DLPchips and used in DLP projector systems. For this application, we preferto use the term DMD digital micromirror device, which may be interpretedbroadly to include any individually controllable array of reflectors,wherein the reflectors are small in size.

FIG. 1 shows a WBC cavity 100. In WBC, an array of laser elements 102 isplaced in an external cavity consisting of a transform lens 104, adiffraction grating 106, and an output coupler 108. Conceptually, WBCcan be thought of as the spatial superposition of many independent diodelaser external cavities. In the example shown in FIG. 1, threeindependent external cavities are shown 110 a, 110 b, and 110 c. Eachcavity has a distinct wavelength characteristic to thatexternal-cavity-stabilized diode laser element. The diffraction grating106 has an angle-to-wavelength conversion property that allows feedbackto each diode laser element in the array, via the transform lens 104, ata different wavelength. Feedback from the output coupler stabilizes eachof the individual emitters. WBC allows for brightness scaling of a diodelaser array because all of the laser elements are spatially overlappedat the output coupler, maintaining the output beam quality of a singleelement while scaling the output power by the number of elements in thearray. Wavelength beam combining can be applied to any laser with a gainbandwidth. For example, these lasers may include diode lasers, fiberlasers, CO2 lasers, and/or Quantum Cascade Lasers (QCLs).

Wavelength beam combining (WBC) is an incoherent process and, thus, doesnot require phasing of laser elements. In some embodiments, thebrightness of the output beam 112 scales proportionally to the totalnumber of laser elements. The output beam 112 of a WBC system is that ofa single beam. In both coherent and WBC systems, the output beam qualityis the same as that of a single emitter but the output power scales thepower from all the laser elements. If both very high spectral brightness(single frequency operation) and very high spatial brightness (singlespatial mode) is required then coherent beam combination is the onlymethod. However, in many cases single frequency operation is not desiredand may be detrimental to the functionality of the system, thus makingWBC the preferred approach.

FIG. 2 illustrates at least one embodiment of the current disclosure.The widely tunable IR cavity 200 consists of an individually addressablediode or QCL laser array 202, a first optical element 210, a diffractiongrating 220, a second optical element 212, an array of individuallyaddressable micromirrors 235 such as a DMD 230, a third optical element214, a reflective mirror 216, and a fourth optical element 240. Theoptical elements 210, 212, 214, and 240 may be transform lenses orcollimating lenses having focal power along one or more planes. Forexample, 210 may cause individual beams emitted from 202 to convergeonto grating 220. Optical element 240 may be used as a collimation lensfor the stabilized output beams 250.

As shown, beams are emitted from 202 and converge onto 220. It should beunderstood that 210, which causes the beams to be angled individuallyonto grating 220 may be absent. In other embodiments, where eachindividual emitter is mechanically positioned to converge on to grating220, this still allows for the angle-to-wavelength conversion propertyof grating 220 to provide feedback into each mechanically positionedemitter at a different wavelength.

After the beams are caused to converge onto grating 220 orders ofdiffracted light occur. In one instance the 0th order beam is used tore-image onto a mirror 216. The reflected beam is overlapped onto thediffraction grating. The output beam is then taken off the 1st order incombination with a chief-ray collimation lens (this would be 240 in FIG.2). The chief-ray collimation lens enables the output to beco-boresighted. A co-boresighted system is important for manyapplications such as various spectroscopy systems including:conventional absorption spectroscopy of trace chemical and biologicalagents, improvised explosive detection, differential absorption lidar(DIAL), and multi-wavelength photo-acoustic spectroscopy, materialverification, anti-counterfeiting, and threat screening.

As mentioned the orders that are diffracted from 220 in the tunablecavity system 200 may be recycled and used as feedback mechanisms tostabilize the individual emitters of 202. Lenses 212 and 214 assist incollimating the diffracted beams and upon being reflected cause thereflected beams to converge back onto the grating. Reflective mirror 216is such a mirror that helps overlap and recycle these orders.

In some embodiments, the cavity 200 may be a conventional WBC. In suchembodiments, discrete wavelength tuning of each element may be possible.At any given time there may be only one beam exiting hitting the DMD230, which is accomplished by switching off all but one of the DMDmirrors 235.

In some embodiments, the cavity 200, may act as a conventional WBCcavity with the exception of not extracting the output beam from DMD235. To illustrate this point assume in one exemplary embodiment, theQCL or diode array 202 consists of 20 emitters, each emitter having apreviously specified gain peak ranging from 6 μm-10.0 μm. Eachsubsequent element in between has a gain peak incrementing by 0.2 μm.For example, in such embodiments, the middle element has a gain peak at7 μm and the last element has a gain peak at 10 μm. If the middle mirrorof the DMD 230 is turned on and all other DMD mirrors 235 are turnedoff, then all 20 elements will lase at the unique wavelength at anincrement of 0.2 μm. For example, in such embodiments the wavelengthsare 6 μm, 6.2 μm, 6.4 μm . . . 9.6 μm, 9.8 μm, and 10.0 μm.

Consistent with the present disclosure, are systems wherein if the leftmost DMD mirror is turned on and all other DMD mirrors are turned offthen the first emitter will lase at a wavelength of 5.9 μm, the adjacentelements will lase at wavelengths in increments of 0.2 μm, and the lastelement will lase at 9.9 μm. In FIG. 2, this is shown schematically asthe left most beam between the grating 220 and the DMD chip 230. If theright most DMD mirror is turned on and all of other DMD mirrors areturned off then the first emitter will lase at 6.1 μm and the adjacentelement will lase at wavelengths in increments of 0.2 μm, and the lastelement will lase at 10.1 μm. In FIG. 2, this is shown schematically asthe right most beam between the grating 220 and DMD 230. Thus, inprinciple any wavelength can be accessed by simply switching the properDMD mirror and turning on the proper diode element. The total number ofDMD mirrors is dictated by the total number of wavelengths per unitdiode (QCL) element. For example, in embodiments where 100 wavelengthsper element are desired at least 100 DMD mirrors are required. In suchembodiments conventional DMD chips may be used. Higher numbers ofwavelengths are also contemplated herein. Turning on any wavelength canbe accomplished very quickly since the settling time of the DMD mirrorcan be as fast as 10 μs.

Contemplated herein are methods to extract higher amounts of useableoutput power. In at least one embodiment, extracting higher amounts ofuseable output power can be achieved by inserting a beam splitter insidethe cavity.

In the embodiment illustrated in FIG. 2 the 0th order output beam 250 isused to re-image onto a mirror 216. In this embodiment, the reflectedbeam is overlapped onto the diffraction grating 220. The output beam istaken off the 1st order in combination with a chief-ray collimation lens240. In some embodiments, regardless of the wavelength, a chief-raycollimation lens 240 may ensure that the output beam 250 is co-boresighted.

Further in FIG. 2, the embodiment depicted includes a QCL array of 20emitters and is 10 mm wide. In embodiments having a grating dispersionof 150 lines per mm, then the required transform lens 210 has a focallength of about 10 mm. This optical setup of this embodiment willdictate a total optical bandwidth of about 4000 nm. The second transformlens 212 and grating 220 help dictate the tuning of each emitter. Forexample, in embodiments where each emitter is tunable to about 200 nmand the pitch of the DMD is 10 μm, the focal length of the secondtransform 212 lens is 5 mm. Thus, the total optical path length from thediode array to the DMD chip, in this embodiment is very compact in size.In at least one embodiment the total optical path length from the diodearray to the DMD chip is about 30 mm.

Consistent with the present disclosure are systems having a gratingelement. In at least one embodiment, a transmission grating may bepreferable, while in other embodiments, a reflection grating may bedesired.

FIG. 3 depicts an efficiency curve of a system, in accordance with thepresent disclosure, having a reflection grating. The efficiency of thegrating is >90% from 5-12 μm.

Consistent with the present disclosure are systems having individuallyaddressable array of quantum cascade laser (QCL) elements. In suchembodiments, electric tuning may be accomplished by turning on only oneelement of the array at a time. Each laser element may be wavelengthlocked to a unique wavelength that is linearly chirped in the array.Thus, in such embodiments, wavelength tuning over the entire 6-10 μmrange may be accomplished and the single output beam would the samecharacteristics and beam quality as a single element that is turned on.Due to the nature of wavelength tuning disclosed. In such embodiments,within a given element, the wavelength shift may be about 200 nm and thetotal bandwidth of the system may 4000 nm. As a result, it iscontemplated that a smearing of the near field may occur at about 200nm/4000 nm=5%.

Quantum Cascade Laser (QCL) Sources

In order to meet the broad wavelength coverage requirement for 6 to 10μm, as described for at least one embodiment above, QCLs having a tuningrange of 100 to 200 nm per QCL may be desired. A spectral bandwidth of200 nm may be supported by the tuning range or gain bandwidth of thelaser element. FIG. 4 shows a typical tuning range of a 5 μm-wavelengthquantum cascade laser which tunes between 5050 nm and 5500 nm, thushaving about a 500-nm tuning range. However, in additional embodiments amore usable tuning range is about 250 nm or 80% of maximum power. Thetuning range scales with the wavelength and may become larger in therange of 6-10 μm. In additional embodiments, a single QCL wafer can begrown epitaxially to cover the wavelength range of 6-10 μm by employingmultiple gain regions with different center wavelengths within the samelaser structure.

In at least one embodiment, as many as 40-50 QCLs may be used to coverthe desired wavelength range. Redundancy of QCLs may be used in someembodiments to help ensure reliable operation. With 40 QCLs for example,the tuning step size may be 100 nm.

In some embodiments, lasing sources may be single emitter, singletransverse mode semiconductor QCLs. In order to obtain a desired powerand a diffraction limited output power, single emitter diodes may beused and mounted on a common heat-sink. In at least one embodiment, thediode may be mounted on a heat-sink using discrete device packagingtechnology; however other mounting technologies commonly known in theart are also consistent with the present disclosure. In at least oneembodiment, each device is lensed with collimating optics. FIGS. 5A-Bdemonstrate a single element power and efficiency Quantum Cascade Lasers(QCLs) having 5.1 W CW (8.3 W pulsed) room temperature QCLs at 4.9 μmwavelength in a single spatial mode. The maximum wall plug efficiency(WPE) under CW operation is 21% and under pulsed operation the maximumWPE is 27%. FIG. 5A shows L-I-V characteristics under CW and pulsedoperation while FIG. 5B shows wall-plug efficiency under CW and pulsedoperation.

As shown in FIGS. 6A-B, in some embodiments, high power, efficient QCLsat longer wavelengths in the LWIR, including 10 microns, may also beused. The high power QCL of this embodiment, obtains up to 25 W in peakpower and >0.5 W under room temperature, CW operation. The device maystill operate under CW operation at a temperature of 45 C.

Digital Light Processing (DLP Aka DMD) Chip

In some embodiments, the fine tuning within each 100 nm band may beaccomplished using a tunable component in the WBC external cavity. Thespecification that drives this requirement is the tuning step time of125 sec (threshold) and 31 psec (objective). One can select a DLP chipthat allows for this very fast tuning. See FIG. 2 for the implementationof the entire design, with the individually addressable QCL array forthe coarse wavelength tuning (100 nm resolution) and the fast DLP arrayfor fine wavelength tuning (0.5 nm resolution).

In at least one embodiment, the DLP chip is a MEMS-based device and hasno large mechanical moving parts.

Control Electronics and Software

In some embodiments, control electronics and software may be used toapply current to the individually addressable QCL array and operate theDMD chip as required for the electronic wavelength tuning. In suchembodiments, the QCLs may operate under pulsed operation, operated by apulsed QCL driver. In some embodiments, the control software may havewavelength sweep modes, ramp modes, and/or any other modes commonly usedin the art.

In at least one embodiment, coarse wavelength tuning may be accomplishedby switching the specific QCL of interest in the array. In additionalembodiments, fine wavelength tuning may be accomplished by adjusting theDMD mirror corresponding to that particular device. By adjusting the DMDmirror, electrical power may be applied to all elements of the QCL arrayconstantly, and wavelength tuning may be accomplished by adjusting theDMD mirror for feedback to a single element within the QCL array.

Although the focus of this application has been on the MID-IR range, theprinciples may apply to wavelengths outside of those ranges that aredetermined by the emitters and gratings used.

The above description is merely illustrative. Having thus describedseveral aspects of at least one embodiment of this invention includingthe preferred embodiments, it is to be appreciated that variousalterations, modifications, and improvements may readily occur to thoseskilled in the art. Such alterations, modifications, and improvementsare intended to be part of this disclosure, and are intended to bewithin the spirit and scope of the invention. Accordingly, the foregoingdescription and drawings are by way of example only.

1.-19. (canceled)
 20. A method of tuning a beam source, the methodcomprising: providing a plurality of beam emitters each emitting a beamhaving a different wavelength; converging the beams emitted by the beamemitters onto a grating, whereby the beams are split into a plurality oforders; reflecting a first one of the orders through the grating andback to the plurality of beam emitters, whereby each beam is reflectedback to the beam emitter from which it was emitted to thereby stabilizethe beam to its different wavelength; receiving a second one of theorders at an array of individually controllable reflectors; andredirecting one or more of the beams within the second one of theorders, with one or more of the reflectors, back to the grating, wherebythe one or more of the beams are transmitted as one or more tuned outputbeams.
 21. The method of claim 20, wherein the beams emitted by the beamemitters are converged onto the grating at least in part by an opticalelement.
 22. The method of claim 20, wherein convergence of the beamsonto the grating arises at least in part due to positioning of beamemitters to emit beams that converge toward each other.
 23. The methodof claim 20, wherein the grating comprises a reflective diffractiongrating or a transmissive diffraction grating.
 24. The method of claim20, further comprising redirecting one or more of the beams within thesecond one of the orders, with one or more of the reflectors, to a beamdump.
 25. The method of claim 20, further comprising collimating one ormore of the tuned output beams.
 26. The method of claim 20, wherein thefirst one of the orders is reflected back to the plurality of beamemitters by a mirror, the mirror being spaced apart from the grating.27. The method of claim 20, wherein the array of individuallycontrollable reflectors is at least a portion of amicro-electro-mechanical systems device.
 28. The method of claim 20,wherein the array of individually controllable reflectors is at least aportion of a digital light processing chip.
 29. The method of claim 20,wherein at least one of the beam emitters comprises a quantum cascadelaser source.
 30. A method of tuning a beam source, the methodcomprising: providing a plurality of beam emitters each emitting a beamhaving a different wavelength; converging the beams emitted by the beamemitters onto a grating; after converging the beams onto the grating,reflecting a first portion of each beam back to the beam emitter bywhich it was emitted to thereby stabilize the beam to its differentwavelength; after converging the beams onto the grating, for at leastone of the beams, reflecting a second portion of the beam, with one of aplurality of individually controllable reflectors, as a tuned outputbeam in a direction away from the plurality of beam emitters.
 31. Themethod of claim 30, wherein the beams emitted by the beam emitters areconverged onto the grating at least in part by an optical element. 32.The method of claim 30, wherein convergence of the beams onto thegrating arises at least in part due to positioning of beam emitters toemit beams that converge toward each other.
 33. The method of claim 30,wherein the grating comprises a reflective diffraction grating or atransmissive diffraction grating.
 34. The method of claim 30, furthercomprising, for at least one of the beams, reflecting a second portionof the beam, with one of the plurality of individually controllablereflectors, to a beam dump.
 35. The method of claim 30, furthercomprising collimating the tuned output beam.
 36. The method of claim30, wherein the first portion of each beam is reflected back to theplurality of beam emitters by a mirror, the mirror being spaced apartfrom the grating.
 37. The method of claim 30, wherein the array ofindividually controllable reflectors is at least a portion of amicro-electro-mechanical systems device.
 38. The method of claim 30,wherein the array of individually controllable reflectors is at least aportion of a digital light processing chip.
 39. The method of claim 30,wherein at least one of the beam emitters comprises a quantum cascadelaser source.